PASCO SP-9268A STUDENT SPECTROMETER - Manual (Page 8)

4
Student Spectrometer 012-02135F
¨
2. The mounting pillars of the telescope and collimator
can be rotated by using an Allen wrench to loosen the
screws that attach the pillars to their respective bases.
To loosen the screw for the collimator, the spec-
trometer must be removed from the wood base. 3. To be sure both optical units are square to the axis of
rotation, follow the focusing procedure described
above, adjusting the mounting pillars as necessary so
the slit image is well centered in the viewing field of
the telescope.
Measuring Angles of Diffraction
When analyzing a light source, angles of diffraction are
measured using the vernier scales. However, the scales
only measure the relative rotational positions of the tele-
scope and the spectrometer table base. Therefore, before
making a measurement, it's important to establish a ver-
nier reading for the undeflected beam. All angles of dif-
fraction are then made with respect to that initial reading
(see Fig 5).
To obtain a vernier reading for the undeflected beam,
first align the vertical cross-hair with the fixed edge of
the slit image for the undeflected beam. Then read the
vernier scale. This is the zero point reading (
θθθθθ
0
).
Now rotate the telescope to align the vertical cross-hair
with the fixed edge of a deflected image. Read the ver-
nier scale again. If this second reading is
θθθθθ
, then the ac-
tual angle of diffraction is
θθθθθ
–
θθθθθ
0
. If the table base is ro-
tated for some reason, the zero point changes, and must
be remeasured.
Reading the Vernier Scales
To read the angle, first find
where the zero point of the
vernier scale aligns with
the degree plate and record
the value. If the zero point
is between two lines, use
the smaller value. In Fig-
ure 6, below, the zero
point on the vernier scale is between the 155 ° and 155 °
30' marks on the degree plate, so the recorded value is
155 °.
Now use the magnifying glass to find the line on the ver-
nier scale that aligns most closely with any line on the
degree scale. In the figure, this is the line corresponding
to a measurement of 15 minutes of arc. Add this value to
the reading recorded above to get the correct measure-
ment to within 1 minute of arc: that is, 155 ° + 15' = 155 °
15'.
LIGHT
SOURCE
ANGLE OF
DIFFRACTION
VERNIER
READING FOR
UNDIFFRACTED
BEAM
q
0
VERNIER READING FOR
DIFFRACTED BEAM
q
q q
0
– =
=
=
VERNIER SCALES
Figure 5 Measuring an Angle of Diffraction
Figure 6 Reading the Vernier Scales
VER I'
0
I0
30 20
I70
I60
I5
155° (on the degree scale) 15' (on the vernier scale)
155° + 15' = 155° 15'